The check in computer often does prompt the requirement for a return ticket within 21 days. I have had issues on at least three occasions, two of them I had non confirmed tickets from my travel agent that I produced and immediately cancelled afterwards. On another occasion boarding Asiana in JFK bound for Cebu I produced my seafarer ID and BS that I was joining a ship, she got her manager involved. The other 100+ times I've flown into the Philippines in the last few years have been fine. Some people purchase cheap low cost airline tickets from the Philippines to Indonesia or Hong Kong that they never use.